Method of trimming an electronic circuit

Abstract

An electronic probe circuit having ac and dc amplifiers and an input compensation subcircuit is enclosed within a trim housing that replicates the electrical effect of the probe housing. The circuit is laser trimmed through ports in the trim housing. The difference between the voltage at 80 nsec and 1.4 μsec points on a step voltage provides a first calibration factor while the difference between the 3 nsec voltage and the 80 nsec voltage provides a second calibration factor. A resistor in the DC amplifier is trimmed to an absolute voltage with a step scan laser cut. A resistor in the AC amplifier is trimmed with a laser L-cut until the difference between the 80 nsec and 1.4 μsec points of the step voltage equals the first calibration factor. A capacitor in the input compensation subcircuit is trimmed until the voltage difference between the 3 nsec and 80 nsec points equals the second calibration factor.

What is claimed is: 
     
       1. A method of trimming an electronic circuit to within a specified amount of a target value of a specified electronic parameter, said method comprising the steps of: (a) applying electrical power to said electronic circuit;   (b) measuring said electronic parameter;   (c) removing individual portions of a resistor in a first direction which is approximately perpendicular to the current flow in said resistor;   (d) repeating steps (b) and (c) until said electronic parameter is within a selected amount of said target value;   (e) removing individual portions of said resistor in a second direction approximately parallel to said current flow; and   (f) repeating steps (b) and (e) until said electronic parameter is within said specified amount of said target value.   
     
     
       2. The method of claim 1 further comprising the step of: allowing sufficient time for said resistor to stabilize before step (b) is performed. 
     
     
       3. A method of trimming an electronic circuit as in claim 2 wherein said steps (c) and (e) comprise removing a series of individual portions of said resistor each of which is small compared to an overall portion of said resistor that is removed by steps (c) and (e). 
     
     
       4. A method of trimming an electronic circuit as in claim 1 wherein said steps (c) and (e) comprise removing individual portions of said resistor of a first predetermined size whenever said electronic parameter differs from said target value by more than a predetermined amount and removing individual portions of said resistor of a second predetermined size whenever said electronic parameter differs from said target value by less than said predetermined amount. 
     
     
       5. The method of claim 4 wherein said first predetermined amount is larger than said second predetermined amount. 
     
     
       6. A method of trimming an electronic circuit as in claim 1 further comprising the steps of: (g) calculating a percentage difference between said measured value of said electronic parameter and said target value;   (h) moving the beam of a laser a first selected distance if said calculated percentage difference is greater than a selected percentage; and,   (i) moving said beam of said laser a second selected distance if said calculated percentage difference is less than said selected percentage.   
     
     
       7. A method of trimming an electronic circuit as in claim 6 wherein said second selected distance is smaller than said first selected distance. 
     
     
       8. A method as in claim 6, said method further comprising the steps of: (c) applying electrical power to said electronic circuit;   (d) taking a first measurement of said electronic parameter;   (e) choosing said length of said first portion based on said first measurement of said electronic parameter;   (f) taking a second measurement of said electronic parameter;   (g) choosing said length of said second portion based on said second measurement of said electronic parameter.   
     
     
       9. A method of trimming an electronic circuit to within a specified amount of a target value of a specified electronic parameter, said method comprising the steps of: (a) removing a first portion of a resistor, said first portion having a length along the direction of current flow in said resistor and a width perpendicular to said direction of said current flow, said width being approximately equal to the width of the kerf of a laser beam;   (b) removing a second portion of said resistor, said second portion starting approximately between the ends of said first portion along said length of said first portion, said length of said second portion being parallel to said length of said first portion and less than or equal to said length of said first portion, said width of said second portion being approximately equal to the width of said kerf of said laser beam.   
     
     
       10. The method of claim 9 wherein step (a) may be repeated more than once and step (b) may be repeated more than once. 
     
     
       11. The method of trimming an electronic circuit or component as in claim 9 further comprising trimming said electronic circuit or component to within a second specified amount of said target value by making an L-cut in said resistor.